In recent image display devices that are becoming widespread, a plurality of cameras attached to a mobile body are used to capture images of the surroundings of the mobile body. The captured images are combined into a bird's eye view or a 360° panorama, and the combined image is displayed. Examples of such image display devices include parking aid systems (around view monitors) for automobiles.
When such an image display device combines the images captured by the cameras, the installation parameters of the cameras are required. Examples of the installation parameters of each camera include its position (X-, Y-, and Z-coordinates) and angles (roll, pitch, and yaw angles) in the vehicle coordinate system.
It is not necessary to very strictly determine the positions in these installation parameters (initial values measured by the user may be used). However, the angles of the cameras installed in the vehicle must be accurately determined by calibrating the cameras because the angles largely affect the final combined image even when the errors in the angles are very small.
Various techniques have been proposed to compute the angles of cameras. For example, in one known technique (see, for example, Japanese Laid-open Patent Publication No. 2008-187566), one calibration jig (calibration pattern) of a known shape is disposed on a road surface at any position in each overlap region of the fields of view of cameras (each common image capturing range of the cameras). The installation parameters of the cameras are computed using the shape conditions of the calibration patterns. Each calibration jig used in this technique has a square shape with sides of about 1 meter to about 1.5 meters, and markers are provided at the four corners of the jig.
In another known technique (see, for example, Japanese Laid-open Patent Publication No. 2008-187564), some of the installation parameters of the cameras are computed in advance by a predetermined method, and two small calibration jigs each having one characteristic point are disposed in each common image capturing range of cameras. The rest of the installation parameters are computed under the conditions in which each camera captures an image containing four markers.
However, the conventional techniques described above have a problem in that the calibration of the cameras cannot be made in a narrow space.
For example, when square calibration patterns are placed to make the calibration of the cameras, at least two calibration patterns with sides of about 1 meter to about 1.5 meters must be disposed in each common image capturing range. Therefore, to compute the installation parameters of the cameras, a space of 2 meters to 3 meters must be provided on each side of the vehicle in addition to the footprint of the vehicle.
Even when small calibration jigs are placed to make the calibration of the cameras, two calibration jigs spaced apart from each other by a certain distance must be disposed in each common image capturing range of cameras, and therefore the calibrations of the cameras cannot be made in a small space. With this technique, some of the installation parameters must be computed in advance by a different method, and this technique alone cannot compute all the installation parameters of the cameras.
Therefore, one important object is to allow the calibration of the cameras to be made even in a narrow space to determine the installation parameters of the cameras accurately.